An ethnic group is a human population whose members identify with each other, usually on the basis of a presumed common genealogy or ancestry (Smith, 1986). Recognition by others as a separate ethnic group, and a specific name for the group, also contribute to defining it. Ethnic groups are also usually united by certain common cultural, behavioural, linguistic and ritualistic or religious traits. In this sense, an ethnic group is also a cultural community. Processes that result in the emergence of such a community are summarized as ethnogenesis.

Members of an ethnic group generally claim a strong cultural continuity over time, although some historians and anthropologists have documented that many of the cultural practices on which various ethnic groups are based are of recent invention (Friedlander 1975, Hobsbawm and Ranger 1983, Sider 1993). On the political front, an ethnic group is distinguished from a nation-state by the former's lack of sovereignty.

While ethnicity and race are related concepts (Abizadeh 2001), the concept of ethnicity is rooted in the idea of social groups, marked especially by shared nationality, tribal affiliation, genealogy, religious faith, language, or cultural and traditional origins, whereas race is rooted in the idea of a biological classification of Homo sapiens according to chosen genotypic and/or phenotypic traits, and a belief that such differences among human beings are of such a magnitude as to be classified by the anthropological sense of "race", i.e. subspecies.

In the West, the notion of ethnicity, like race and nation, developed in the context of European colonial expansion, when mercantilism and capitalism were promoting global movements of populations at the same time that state boundaries were being more clearly and rigidly defined. In the nineteenth century, modern states generally sought legitimacy through their claim to represent "nations." Nation-states, however, invariably include populations that have been excluded from national life for one reason or another. Members of excluded groups, consequently, will either demand inclusion on the basis of equality, or seek autonomy, sometimes even to the extent of complete political separation in their own nation-state.

Sometimes ethnic groups are subject to prejudicial attitudes and actions by the state or its constituents. In the twentieth century, people began to argue that conflicts among ethnic groups or between members of an ethnic group and the state can and should be resolved in one of two ways. Some, like Jürgen Habermas and Bruce Barry, have argued that the legitimacy of modern states must be based on a notion of political rights of autonomous individual subjects. According to this view the state ought not to acknowledge ethnic, national or racial identity and should instead enforce political and legal equality of all individuals. Others, like Charles Taylor and Will Kymlicka argue that the notion of the autonomous individual is itself a cultural construct, and that it is neither possible nor right to treat people as autonomous individuals. According to this view, states must recognize ethnic identity and develop processes through which the particular needs of ethnic groups can be accommodated within the boundaries of the nation-state. This is the nationalist viewpoint.

In English, Ethnicity goes far beyond the modern ties of a person to a particular nation (e.g., citizenship), and focuses more upon the connection to a perceived shared past and culture. See also Kinship and descent, Romanticism, folklore. In some other languages, the corresponding terms for ethnicity and nationhood may be closer to each other.

The nineteenth century saw the development of the political ideology of ethnic nationalism, when the concept of race was tied to nationalism, first by German theorists including Johann Gottfried von Herder. Instances of societies focusing on ethnic ties to the exclusion of history or historical context arguably have resulted in almost fanatical justification of nationalist or imperialist goals. Two periods frequently cited as examples of this are the nineteenth century consolidation and expansion of the German Empire, and the Third Reich, each promoted on the theory that these governments were only re-possessing lands that had "always" been ethnically German. The history of late-comers to the nation state model, such as those arising in Near East and southeast Europe out of the dissolution of the Ottoman and Austro-Hungarian Empires, as well as those arising out of the former USSR, is particularly marked by inter-ethnic conflicts.

The Human Genome Diversity Project (HGDP) has attempted to map the DNA that varies between humans, which is a less than 1 % difference. This data could create definitive proof of the origin of individual ethnic groups.